The vertebrae of the human spine are arranged in a column with one vertebra on top of the next. Between each vertebra exists an intervertebral disc that transmits force between adjacent vertebrae and provides a cushion between the adjacent vertebrae.
Sometimes, back pain is caused by degeneration or other deformity of the intervertebral disc (“diseased disc”). Conventionally, surgeons treat diseased discs by surgically removing the diseased disc and inserting a bone graft in the space vacated by the diseased disc. The adjacent vertebrae are then immobilized relative to one another. Eventually, the vertebrae grow into one solid piece of bone.
Currently, it is difficult to insert the bone graft into the vacated space and fuse the adjacent vertebrae. The current process of inserting a bone graft and fusing the adjacent vertebrae will be explained with referring to FIGS. 1 and 2. FIG. 1 shows two adjacent vertebrae 102 and 104. Located between vertebrae 102 and 104 is an intervertebral space 106 partially filled by a bone graft 108. When the bone graft 108 is first inserted into the intervertebral space 106, the adjacent vertebrae 102 and 104 are manually kept apart by the surgeon using, for example, a retracting device (not shown). As shown in FIG. 2, once the bone graft 108 is placed, the surgeon releases the adjacent vertebrae 102 and 104 allowing them to squeeze the bone graft 108 and hold the bone graft 108 in place.
To immobilize the vertebrae 102 and 104 with the bone graft 108 in place, the surgeon next applies a cervical plate 202 over the adjacent vertebrae 102 and 104. Cervical plate 202 may have a central viewing window 204 and one or more screw holes 206, in this example four screw holes 206a-206d are shown. Four bone screws (which will be identified by reference numerals 208a-208d) would be screwed into the vertebrae using the screw holes 206 to anchor the cervical plate to the vertebrae and immobilize the vertebrae with respect to one another.
As can be appreciated, attaching the cervical plate 202 using the bone screws 208 is a difficult endeavor. Generally, a temporary screw (not shown) is placed in one of the screw holes, for example 206a. Bone screw 208c would then be partially screwed into the bone at screw hole 206c. The temporary screw in hole 206a would be replaced by bone screw 208a, which would be tightened. Then the other bone screws 208 would be screwed into the bone in a cross point manner. The ability of the cervical plate to move freely in relation to the vertebrae 102 and 104 and the bone graft 108 until the bone screws anchor the plate causes difficult in attaching the cervical plate. This is made more difficult because, generally, only a portion of the cervical plate is visible to the surgeon at any given moment (due to space constraints and surgical tools).
Moreover, once threaded, bone screws 208a-d tend to backout or reverse thread. Current fusion plates have numerous backout devices, but those devices are less than satisfactory solutions in some instances. Thus, improved backout or reverse threading protection is desired.